Holding Back Floodwaters With A Balloon

New York Times :
November 19, 2012

MORGANTOWN, W.Va. — With a few dull thuds, the 1-ton bag of high-strength fabric tumbled from the wall of the mock subway tunnel and onto the floor. Then it began to grow.

As air flowed into it through a hose, the bundle inflated until it was crammed tight inside the 16-foot-diameter tunnel, looking like the filling in a giant concrete-and-steel cannoli.

The three-minute procedure, conducted on a chilly morning this month in an airport hangar not far from West Virginia University, was the latest test of a device that someday may help guard real tunnels during disasters — whether a terrorist strike or a storm like Hurricane Sandy, whose wind-driven surge of water overwhelmed New York City's subway system, shutting it down for days.

The idea is a simple one: rather than retrofitting tunnels with metal floodgates or other expensive structures, the project aims to use a relatively cheap inflatable plug to hold back floodwaters.

In theory, it would be like blowing up a balloon inside a tube. But in practice, developing a plug that is strong, durable, quick to install and foolproof to deploy is a difficult engineering task, one made even more challenging because of the pliable, relatively lightweight materials required.

“Water is heavy; there's a lot of pressure,” said Greg Holter, an engineer with Pacific Northwest National Laboratory who helps manage the project. “So it's not as simple as just inflating and filling the space. The plug has to be able to withstand the pressure of the water behind it.”

The idea has been in development for more than five years — this test was the 21st — and Fortune says there are at least a few more years of testing and design work ahead.

If the plugs are shown to be effective, they will be made available to transit systems around the country.

In all, seven of New York City's 14 under-river subway tunnels were flooded during the storm, as were several major highway tunnels. Fortune said that plugs might work for highway tunnels, too, but that the larger size of those tunnels created additional technical challenges that would have to be overcome.

Work on the plug began in 2007, after Ever J. Barbero, a West Virginia professor whose specialty is the use of advanced materials in engineering, was contacted by a Homeland Security official looking for outside-the-box ideas on ways to keep a subway system from flooding if an underwater tunnel were breached — by a terrorist bomb, for example.

Barbero came up with an idea and shared it with Homeland Security officials.

“I said, ‘We'll put an air bag in a tunnel,'” he recalled. The department decided to finance the project. About $8 million has been spent so far.

“It took time to figure out how to make it work with such huge pressure,” Barbero said. “But it also took time for the team to understand how a subway works, what are the constraints in the tunnels.”

A subway tunnel is hardly a pristine environment; it is full of grease and grime — and, often, rats.

There also are obstructions like tracks, as well an electrified third rail, pipes and safety walkways, all of which could cause gaps between the plug and the tunnel walls. Most can be dealt with by modifying a short section of the tunnel to accommodate the plug, which is 32 feet long when inflated. Sharp corners can be curved, flush tracks of the type used at grade crossings can be installed, the third rail can be discontinued for a stretch, and pipes can be made to swing against the ceiling.

Those modifications will reduce potential gaps but not eliminate them. In the most recent test, when Barbero and a colleague, Eduardo Sosa, inspected the front of the plug, they discovered a two-inch gap in one corner. The procedure called for filling the plug with water to pressurize it further, and then introducing water behind it to simulate a flood. But a plumbing failure, unrelated to the plug, ended the test prematurely. It was repeated successfully several days later, Fortune said, and the plug held back all but a small amount of water.